The word “light” is intended—in the wide sense and includes, in particular, spectral bands in the infrared as will be seen below, a major application of the invention being to filter light in the various fiber-optic telecommunication bands lying between 1.3 and 1.61 micrometers.
The advantage of these 1.3 to 1.61 micrometer bands results from the fact that current optical fibers, made of glass, have low attenuation and the optical signals can therefore be transmitted over large distances. In what follows, the invention will be explained with reference to this spectral band, although it should be understood that the invention may be applied to other bands if the need arises, by using materials suitable for these different bands.
In a fiber-optic telecommunication network, a cable comprising a plurality of optical fibers can be used to form a plurality of different transmission channels; time division multiplexing of the information may also be carried out in order to achieve the same objective; with a view to further increasing the information delivery capacity of the network, however, the current trend is for a plurality of light wavelengths, modulated independently of one another and each defining an information channel, to be transmitted simultaneously on the same optical fiber. ITU (International Telecommunications Union) Standard 692 proposes the definition of adjacent channels with an optical spectral bandwidth of 100 GHz, centered on N adjacent standardized optical frequencies whose values are 200 terahertz, 199.9 terahertz, 199.8 terahertz, etc., corresponding to N wavelengths of from 1.52 micrometers to 1.61 micrometers. Modulation of the light at from 10 to 20 gigabits per second can be carried out on a channel having this bandwidth, without too much risk of interference between the immediately adjacent spectral bands (by using modulation pulses of Gaussian shape in order to minimize the passband occupied by this modulation). This technique of frequency division multiplexing is referred to as DWDM, standing for “Dense Wavelength Division Multiplexing”.
In a telecommunication network, the problem is therefore that of being able to collect the light corresponding to a determined channel without perturbing the light of the neighboring channels. At a transmission node of the network, which is assigned to the transmission and reception of information on channel i, for example, it is necessary to be able to collect the light at the central frequency Fi (wavelength λi) without impeding transmission of the light modulating the central frequencies F1 to FN, even though these optical frequencies are very close together.
To that end, there is a need to produce optical filtering components which are highly selective for light wavelengths and are capable of transmitting the central optical frequency Fi and the frequencies lying in a narrow band of less than 50 GHz on either side of this frequency, and of blocking the other bands. Only the light of channel i is collected at the output of such a filter, and this can be demodulated in order to collect the useful information.
It has already been proposed to produce filtering components that operate on the principle of Fabry-Pérot interferometers, which are produced by depositing semiconductor layers separated from one another by air gaps with thicknesses calibrated according to the wavelength λi to be selected. In practice, an interferometer comprises two mirrors made of stacked dielectric layers (Bragg mirrors) with a high coefficient of reflection, which are separated by a transparent zone with an optical thickness of k·λi (real thickness k·λi if the zone is an air gap), where k is an integer defining the order of the interferometric filter. Indium phosphide (InP) is highly suitable for these embodiments, in particular because of its transparency for the wavelengths in question, its very high refractive index and the possibility of depositing epitaxial layers with a well-controlled thickness.
If the thicknesses of the layers and the intervals between layers are very well controlled, and if the materials have a high refractive index, such a filter turns out to be highly selective.
Such an embodiment is described in the article by A. Spisser et alii, “Highly Selective 1.55 micrometer InP/airgap micromachined Fabry-Pérot filter for optical communications” in Electronics Letters, No 34(5), pages 453–454, 1998. Other embodiments, made of micromachined silicon and of alloys based on gallium arsenide, have been proposed.
These filters may be tunable by varying the thickness of the Fabry-Pérot resonant cavity, that is to say the transparent zone separating the two mirrors. The cavity is delimited by two opposing semiconductor layers, the spacing of which is defined very precisely during fabrication; by making an electrical contact on each of them (the layers being assumed to be sufficiently conductive or coated with a conductive material), a DC voltage can be applied that creates electrostatic forces between the opposing layers, tending to modify the spacing in a controlled way.
It has been shown that it is possible to produce interferometric filters whose layers are suspended from micromachined suspension arms with a thickness small enough so that a voltage of a few volts is sufficient to modify the tuning of the filter over about one hundred nanometers, for example throughout the wavelength range of the standardized band of from 1.52 to 1.61 micrometers. The nominal thickness of the cavity is 0.785 micrometers, for example, and applying a voltage of the few volts makes it possible to vary this thickness by 100 or 200 nanometers up or down, which is sufficient to modify the tuning of the filter throughout the spectral band of the ITU standard. In practice, the tuning may be modified by 1 GHz per millivolt, which is very satisfactory since it is then possible to change channel with a control voltage modification of from 50 to 100 millivolts.
It is, then, sufficient to establish a correspondence table between a channel number (and therefore a central wavelength) and a control voltage to be applied for selecting any one of the channels, and to send the output of the filter to a demodulator (photodetector) which converts the information carried by this channel into an electrical signal.
It will, however, be understood that the tuning conditions would be easy to control if the central frequencies were far apart, for example separated by 1 terahertz, but that they are much more difficult to control when the spacing is only 100 gigahertz. This is because even at transmission, the frequency of a laser which emits the carrier of channel i may experience fluctuations and drifts due to temperature or to ageing (of the order of a few tens of gigahertz).
It is therefore desirable to slave the control voltage of the filter once it has been locked onto the correct central frequency, in order to maintain this voltage subsequently.